EN
We used a non-invasive TOBEC method (Total Body Electric Conductivity) to estimate lean body mass and fat content in mouse-eared bats Myotis myotis (Borkhausen, 1797) hibernating in Poznań Forts (W Poland) and in a semi-natural cave-mine Miedzianka (SE Poland). In December, fat content averaged 5.5 g in females (body mass = 29.4 g) and 5.3 g in males (body mass = 28.4 g). At the end of hibernation (April), fat content averaged 2.2 g in females (body mass = 25.6 g) and 1.4 g in males (body mass = 23.7 g). Fat content did not differ between the localities either in December or in April, but the pattern of changes of fat content was different. We calculated the rate of energy expenditure in hibernating bats using two methods, based on independent samples (fat content in first-time captured individuals) and based on paired observations (changes of fat content in re-captured individuals), and discussed problems associated with the two approaches. Both methods show that the bats need about 4.9 g of fat (191 kJ) to sustain a 165-day hibernation. However, the rate of fat usage varied considerably between the sites and hibernation phase. Although the average amount of fat remaining in April would be sufficient to support at least six more weeks of hibernation, the level of reserves was close to zero in some individuals.